1. Field of the Invention
The present invention relates to a method of cleaning of an inorganic filler in resin-based materials, and to a method for surface treatment of the inorganic filler after cleaning. The proposed methods should be applied resin-based materials for industrial or biological use, particularly dental resin restorative materials such as dental resin composite, resin-based adhesives, cements, resin-made tooth crown prostheses, and denture base resins, in order to bind an inorganic filler firmly to a resin matrix.
2. Description of the Conventional Art
Dental resin restorative materials represented by resin composite and resin-based adhesives, cements have been widely used in dentistry. These materials are generally composed of inorganic filler particles blended into a resin matrix, as a substrate to enhance the strength of the dental resin restorative after curing. As a method for enhancing the strength of the dental resin restorative, is employed a method in which in order to bind an inorganic filler to be formulated into the dental resin restorative materials more firmly to a resin matrix, the inorganic filler is subjected to a silane coupling processing.
In the inorganic filler used for the dental resin restorative material, the strength and surface properties of the dental resin restorative after curing vary depending on the particle size thereof. Although, inorganic fillers having a particle size of from several tens .mu.m to several hundreds .mu.m have been used for the conventional dental resin restorative materials, in recent years, microfine inorganic fillers having a particle size of about 0.04 to 5 .mu.m are mainly used in the dental resin restorative material, resulting in increase of a specific surface area of the inorganic filler in the dental resin restorative material.
It has been indicated that organic substances, such as carbon, which are considered to attach during the production or storage of the inorganic filler, i.e., contaminants on the inorganic filler surface, adversely affect the binding between the inorganic filler and the resin matrix strongly. These contaminants weaken the binding between the inorganic filler and the resin matrix. In addition, they inhibit the binding of a silane coupling agent onto the inorganic filler surface. Thus, a cleaning method for removing surely the contaminants from the inorganic filler is being investigated.
As a conventional cleaning method for inorganic fillers, a cleaning method in which water or a solvent has been generally applied, to remove contaminants by utilizing its dissolving power or diffusing power, has been generally employed. Water, low-boiling point paraffinic hydrocarbons, high-boiling point petroleum hydrocarbons, alcohols, or surfactants are mainly used in this method. However, water has a low dissolving power to oily organic substances, so that sufficient cleaning effects cannot be expected; and although the low-boiling point paraffinic hydrocarbons have a high dissolving power to oily organic substances, they involve problems against ignition or explosion during the use. Also, the high-boiling point petroleum hydrocarbons have such defects that a dissolving power to oily organic substances is low and that their removal and drying after the cleaning are difficult. Further, the alcohols are not effective against oily organic substances, have risks of ignition or explosion during the use, as well as the low-boiling point paraffinic hydrocarbons. Although the surfactants can remove oily organic substances or aqueous stains to some extent, it is difficult to remove the surfactants themselves after the cleaning.
As described above, a special attention is required to the handling for the conventional cleaning methods of inorganic filler. In addition, contaminants on the inorganic filler surface cannot be thoroughly removed, thus, it is pointed out that the binding between the inorganic filler and the resin matrix is insufficient, resulting in low durability of the dental resin restorative used in an oral cavity for a long period. In other words, the strength of the dental resin material is gradually lowered.